It is well known to use at least one image acquisition unit including an image detector, as an analogue camera or a CCD or CMOS technology device, located in a place of the vehicle's exterior, in order to obtain digitized data representative of some captured images by said image detector, being said data susceptible to be treated later on by electronic means to provide, for example, warning signals for the conductor or a rear image shown on a screen.
It is usual to incorporate one of said image acquisition units in a exterior rear view mirror unit of the vehicle, in order to provide digitized data representative of a rear image, apart from the rear image reflected by the reflector. Due to the fact that the image acquisition unit is located in the exterior of the vehicle, although it is protected up to a certain point against humidity by a specific casing and/or a casing of the rear view mirror, it is exposed to ambient and climatic conditions, which lead to well known problems that may leave the image acquisition unit out of order. The most frequent problems are the following:                appearing of a dew, frost or ice layer in an exterior glass of the image acquisition unit due to elevated exterior humidity conditions in combination with low temperatures;        steaming up in different surfaces which form the optical system of the image acquisition unit, due to the presence of humidity in the interior of the casing and to temperature differences between the interior and exterior of the casing;        accumulation of water drops or snowflakes on the exterior glass of the image acquisition unit, due to rain, snow, water sprinkles, etc.;        accumulation of dust, dirt, etc.        
The reflector surface of the rear view mirror is also exposed to the majority of previous stated problems and it is known to incorporate some heating means, for example, in the form of a configuration of serigraphic resistive paste or ink lines on the rear surface of the reflector and connected to a vehicle's direct current power supply, analogue to the one used in the electrically heated rear window.
There is the need to dispose of some means to avoid lacking transparency problems, due to ambient and climatic factors in an exterior glass surface or in the optical system of the image acquisition unit. Nevertheless, neither on the market, nor through any document, no image acquisition unit is known for monitoring the exterior of a vehicle equipped with a heating device for that target.
The aim of the present invention is to provide an image acquisition unit for the monitoring of the exterior of a vehicle, provided with a heating device for providing thermal energy to a transparent element and/or optical system of the unity, in order to avoid lacking transparency problems, due to ambient and climatic factors.